The present invention relates to superconductor quantum interference devices, sometimes referred to as SQUID's, and also to systems including such devices.
SQUID's, refrigerated to near absolute zero (about 4.degree. K. depending on the material), have been found extremely useful as sensors for measuring various quantities, such as magnetic field, magnetic field gradient, voltage, resistance and mutual inductance. These applications are based on the property of such devices of being "weakly" superconducting, in which both London's concept of fluxoid quantization and the Josephson effect combine to produce a highly sensitive and periodic response to magnetic flux. Such devices are capable of producing magnetometers and other electrical measuring instruments of extremely high sensitivity.
SQUID's have also been proposed for use in digital applications, such as switches, flip-flops, and memory cells. These applications are based on their property of displaying two stable states, namely the normal state, and the superconducting state, the devices being switchable by a small magnetic field from one state to the other. The devices involve very low power dissipation, and therefore they enable dense packing. This produces a very substantial reduction in space requirements as well as in the information transit time, making the device potentially very useful in computer applications.
Both the linear and the digital SQUID's are based on two types of "weak link" coupling junctions, namely:
(1) THE Josephson junction, consisting of a thin oxide layer (10-20 A) sandwiched between two superconductors and
(2) THE NARROW MECHANICAL CONSTRICTION OR Dayem bridge, having a width and length of the order of a few microns. Both types, in their present state of development, have a number of serious drawbacks:
Among the drawbacks of the Josephson junction type are its fragility, limited lifetime, and poor resistance to thermal cycling.
The mechanical constriction, or bridge device, since it operates only very close to the critical temperature of the superconductor, has a serious dimensional drawback. This is due to the high critical current density in usual superconductors, of the order of 10.sup.6 - 10.sup.7 A/cm.sup.2, somewhat below the critical temperature, which would require an extremely small cross-section of width-times-thickness, in the order of 10.sup..sup.-11 cm.sup.2, in order to obtain the small critical current of a few micro-amperes necessary for proper operation of the device. Such small cross-sections are today impractical from an industrial point of view.
The foregoing drawbacks have made SQUID's expensive and relatively unreliable, and therefore such devices have so far found very limited commercial use. This is particularly true with respect to computer applications wherein, notwithstanding a considerable investment of research and development effort, no computer including such devices has yet been commercially released.